Following in the ways of his father. It was this news that revived Yaakov’s spirit that his son Yosef was not lost, but was truly alive and living, connected to Torah and following in its ways.
The Kli Yakar notes that the wagons were actually provided by Pharaoh. Why does the Posuk say that Yaakov “saw the wagons that Yosef had sent”. Further, if Yosef was alluding to the Parsha of עגלה ערופה, one wagon would have sufficed.
The Kli Yakar notes that what revived Yosef’s spirit was not seeing the wagons, but rather the fact that Yosef had sent them. Even though the wagons were provided by Pharaoh, Yosef had escorted the brothers as they left Egypt and sent them off on their way home, as the Posuk says “and he sent his brothers and they went”.
Yosef had learned the law of Levaya - escorting someone when they take leave, from his own father. When Yaakov sent Yosef to check on his brothers, he said “go and I will send you to them”. The word “go” would have been sufficient. By adding the words “and I will send you”, we learn that Yaakov escorted Yosef at the start of his trip. Yaakov had learned this law from Avraham who escorted the three angels as they took leave of him.
The requirement to escort a guest who leaves is learned from the Mitzvah of עגלה ערופה. The sages of the city nearest the corpse have to make a declaration that “our hands did not spill this blood”. Certainly the sages did not commit murder. What they are saying is that we were not responsible for the death by sending this person away from our city without food and without escorting him.
This was the Halacha that Yaakov taught Yosef in their last encounter, when Yaakov performed the Mitzvah of Levaya. Presumably Yosef taught the Halacha to his brothers, when he fulfilled the Mitzvah when escorting them.
Yaakov was not revived on seeing that Yosef remembered the Sugya of עגלה ערופה as a theoretical Torah discussion that they had studied together. It was when he saw that Yosef lived what he had learned, practicing what he had studied in deed, that he had internalised the moral lessons of the Torah. Then he knew that Yosef was truly alive.
